A chemical found in everything from babies' dummies to condoms may cause cancer, global health chiefs say.
The World Health Organisation has concluded that MBT, which is used in rubber manufacturing, 'probably causes cancer'.
It says the chemical is in rubber gloves, condoms, soft playground surfaces made of 'rubber crumb', medical catheters and car tyres. Other potential sources include rubber insoles for shoes, air beds, elastic bands, babies' dummies and swimming caps and goggles.
Rubber crumb, granules of rubber made from recycled tyres, is also a key component of the 3G football pitches that are springing up around the country. The all-weather surfaces are already at the centre of a cancer scare, with claims that they contain mercury, lead, arsenic and other toxic chemicals.
It is feared that players, and particularly goalkeepers, will accidentally swallow the substance when diving. The rubber could also become lodged in cuts and grazes.
Nigel Maguire, whose 18-year-old son Lewis, a keen goalkeeper, has the immune system cancer Hodgkin lymphoma, said: 'This crumb rubber goes everywhere.'
Mr Maguire, who took early retirement from his job as chief executive of NHS Cumbria to help his son, said: 'Lewis has come in saying, 'I've got it in my eyes, in my ears, in my mouth'. We'd have to scrape it off his kit. The more I look into it, the more horrified I am.'
At a meeting in Lyon, France, 24 experts from eight countries said they were satisfied there was enough evidence to add the chemical – which has long been blamed for skin allergies – to its 'encyclopaedia of carcinogens'.
The rating, as a substance that 'probably causes cancer', puts it alongside red meat and only one rung below cigarettes, asbestos and other substances that definitely cause the disease.
Professor Hans Kromhout, a member of the committee that reviewed the chemical, said: 'It has been identified in gloves and baby bottle teats and soothers.
'It has recently been identified in inhalable road dust with the wearing of rubber tyres the most likely source of this contamination.'
A leading British expert on chemical exposure urged the public not to worry, saying everyday items were unlikely to pose a threat.
But the WHO admitted that it was unaware of the scale of the risk.
And campaigners urged people to ask retailers if MBT – full name 2-mercaptobenzothiazole – was in products before buying them.
The WHO's International Agency for Research on Cancer reviewed data on the substance, including a study of workers at a Welsh chemical factory. The research, by professor Tom Sorahan, of Birmingham University, linked MBT to bladder cancer, bowel cancer and a type of blood cancer. But because the workers were also exposed to other chemicals, it was hard to be certain MBT was to blame.
But, with other research showing the chemical to raise the risk of cancer in animals, the WHO – outlining its decision in the journal Lancet Oncology – ruled there was a strong possibility it was also harmful to people.
A spokesman said: 'MBT is used mainly in manufacturing rubber products. The most important exposures are to workers in the chemical and rubber industries.
'The general public may be exposed to small amounts of MBT by skin contact with some rubber goods, such as gloves and footwear, or by inhaling tyre dust in urban air. Risks to the public at large from these types of exposures have not been studied.'
Professor Sorahan said large amounts of MBT loose in factories could be dangerous but said the small amounts woven into everyday products were not likely to be harmful. He added: 'I doubt whether MBT is a health issue for the general population but workers manufacturing or using the chemical need to be protected.'
But Dr Michael Warhurst, of campaign group CHEM Trust, said: 'We need the regulatory system to work faster to protect us. People would be right to be concerned about this, and to ask retailers whether this chemical is present in products that they have bought.'